1. Field of the Invention
This invention relates generally to rotary helical screw compressors and more particularly, it relates to capacity control systems for variable and fixed speed helical screw compressors utilized in refrigeration apparatus.
2. Description of the Prior Art
In U.S. Pat. No. 3,924,972 issued to P. G. Szymaszek on Dec. 9, 1975, there is described a control means for a variable capacity rotary screw compressor which includes an automatic control unit to position a slide valve to control compressor capacity as a function of some system condition being monitored.
In U.S. Pat. No. Re. 29,283 issued to D. N. Shaw on June 28, 1977, there is described a rotary, helical screw compressor having an axially shiftable slide valve member in response to the differential pressure between the trapped volume in the compression chamber and the discharge port line pressure.
In U.S. Pat. No. 4,058,988 issued to D. N. Shaw on Nov. 22, 1977, there is described a helical screw rotary compressor having oppositely oriented slide valves at the suction and discharge sides of the machine to control compressor capacity and balance the closed thread pressure at discharge with discharge line pressure in a main closed loop heat pump refrigeration system.
In U.S. Pat. No. 4,147,475 issued to J. C. Shoop et al. on Apr. 3, 1979, there is shown a helical screw air compressor unit having a rotary capacity control valve, a compressor inlet throttling valve, and a pressure relief valve for venting the compressor discharge conduit.
In U.S. Pat. No. 4,151,725 issued to K. J. Kountz et al. on May 1, 1979, there is shown a control system for regulating a centrifugal compressor through the combination of adjustable inlet guide vanes and a variable speed motor for varying the compressor capacity.
As is generally known, a rotary helical screw compressor includes a fixed compressor housing which has a low pressure port at one end and a high pressure port at the other end. The housing has a working chamber in terms of closed threads formed by a pair of parallel intersecting bores in which are rotatably mounted two intermeshing screw rotors. The screw compressor operates as a positive displacement machine for compressing air or gas such as a refrigerant between the suction or low pressure side and the discharge or high pressure side of the screw compressor.
Due to the importance of energy conservation in recent years, there has been needed a way to operate such screw compressors in a more efficient and reliable manner so as to improve efficiency and thus achieving energy cost-savings. It has been discovered that a rotary helical screw compressor of the variable speed type can be operated with improved efficiency not only by varying the compressor capacity through modulating the speed of a variable speed motor driving the compressor, but by adjusting optimally also the volume of gas trapped in the working chamber prior to discharge as a function of the pressure ratio across the compressor. In a rotary helical screw compressor of the fixed speed type, improved efficiency is achieved by varying the compressor capacity through modulating the volume of gas trapped in the working chamber prior to compression and by adjusting optimally the volume of gas trapped in the working chamber prior to discharge as with the variable speed type.